1. Field of the Invention
The present invention relates to a radial pneumatic vehicle tire for which tread surface anomalies causing user dissatisfaction are diminished without decrease in tire performance such as wet traction and braking performance. More specifically, the invention relates to a pneumatic tire having a plurality of axially spaced apart essentially longitudinal grooves separating essentially longitudinal ribs. On at least one of said ribs, transverse grooves or cuts repeat in the circumferential direction to form first and second land portions wherein the first land portions comprise blocks having a circumferential length greater than that of the second land portions.
2. Description of Related Art
In order to improve the wet traction, wet grip, braking performance and the like, radial pneumatic tires have treads with longitudinal or zigzag grooves extending in the circumferential direction, and, for further traction improvement, transverse grooves axially connecting the circumferential grooves to form blocks. To maintain a good level of traction performance, the transverse grooves or cuts need to be present throughout the service life of the tire tread. Unfortunately, to achieve this the tire must have transverse grooves whose depth is substantially equal to the depth of the longitudinal grooves. An example of such a reference tire 100 is shown in FIGS. 1a and 1b, respectively, in a full tire view and a plan view of the tread portion of the tire. In this example the tread blocks 20 are circumferentially spaced apart by the substantially full depth transverse grooves 30. Tire treads so designed are commonly used on the drive axle of vehicles and have acceptable wet traction performance, but are known to have reduced tread rigidity resulting in the formation of tread surface anomalies such as a xe2x80x9cheel-and-toexe2x80x9d or xe2x80x9csawtoothxe2x80x9d profile or tread block depression. These anomalies result in user dissatisfaction due to either unacceptable visual appearance of the tire or ride discomfort caused by tread induced vibrations. Either factor can cause removal of the tire from service prior to delivering its full potential tread service to the user.
To achieve some kind of compromise between ace anomalies and traction performance, tires have been designed having transverse grooves defining blocks 20 where the transverse grooves 30 have a depth d substantially less than the depth h of the longitudinal grooves, an example of which is tire 200 shown in FIG. 2a. The land portions of the tread bounded by edge 22 of a first block 20 and by edge 21 of a second block 20 are commonly referred to as xe2x80x9cbridgesxe2x80x9d. For values of d/h near zero, tires will have poor traction, and for values of d/h approaching unity, tires may develop surface anomalies leading to reduced service life of the original tread. An acceptable result can be obtained when the tire tread is designed so that the ratio R1=d/h of groove depth d to the tread depth h is such that d/h is between about 0.1:1 to about 0.2:1. Unfortunately, tires experience a loss of tread rubber due to factors such as abrasion, fatigue and the like during their service lives. As a result, tires having tread designs such as shown in FIG. 2a, that is with shallow transverse grooves, will wear in such a manner that the ratio d/h will continually decrease and eventually approach a value of zero. The disadvantage of such a tire wherein d/h approaches zero is the aforementioned loss of wet grip, braking performance and the like.
Tests under highway use conditions were conducted on tires such as tire 200 having a new tire tread depth of approximately 20.5 mm with transverse grooves approximately 3 mm deep. The evolution of d/h just described is demonstrated by the test results shown in FIG. 2b which shows the measured tread depth versus circumferential position for a section of the tire. After 54,000 kilometers of service the tread depth has reached an approximate value of 17 mm everywhere, and the ratio of d/h is approximately zero. In this case the tires are more often removed from service for a perceived loss of traction rather than for the onset of surface anomalies. In an effort to mitigate this counterperformance, tire designers often add additional siping or employ complex block geometry which, instead of improving the situation, may further generate surface anomalies and/or sensitivity to chipping or tearing. Thus a tire tread design that maintains the optimum value of the ratio d/h throughout the service life of the tread is needed.
An object of the present invention is to provide an improved radial pneumatic tire which maintains good wet traction performance and is free of surface anomalies. This object is obtained by a tread portion of the tire having a plurality of longitudinal grooves which form ribs and at least one of those ribs being transected by narrow transverse groove or cuts which form alternating land portions wherein the first land portions are longer than the second land portions.
According to the notation shown in FIG. 3b for tire 300, the first land portion will hereafter be referred to as block 20 and the second land portion as sacrificial bridge 30. An object of the invention is to maintain a non-zero value of the ratio R1=d/h. To accomplish this object, the sacrificial bridge must be decoupled from the adjacent tread blocks 20. This decoupling can be achieved by narrow transverse grooves or cuts 40 and 50. Cut 40 is located between the trailing edge 22 of a first of blocks 20 and a leading edge 31 of the sacrificial bridge 30. Cut 50 is located between the trailing edge 32 of the sacrificial bridge 30 and the leading edge 21 of a second of blocks 20. Leading and trailing edges are defined relative to the rolling direction of the tire with the leading edge 21 being the first point on block 20 to engage the ground during rolling of the tire and the trailing edge 22 being the last point on block 20 to engage the ground during rolling of the tire. The sacrificial bridge 30 is bounded in its circumferential extent by cuts 40 and 50 and in its lateral extent by circumferential grooves 10. The depth h1 of the cuts 40 and 50 and the height h2 of the sacrificial bridge 30 are such that the surface 33 of the sacrificial bridge 30 contacts the ground during rolling of the tire under load. An example of such a design is the tire 300 shown in plan view in FIG. 3a. 
Since the surface 33 contacts the ground when rolling under load, the sacrificial bridge 30 will be subjected to longitudinal shearing forces during the period of ground contact. This shearing force must be sufficient to generate a rate of rubber loss (measured in mm/km) from the sacrificial bridge such that the ratio d/h is maintained. In order to solve the problems found in reference tires, the inventor has found that an optimum level of shearing force, and thus, rate of rubber loss will be obtained only for certain ranges of the values of R1=d/h, R2=h1,/h, and the ratio of sacrificial bridge length L2 to block length L1, R3=L2/L1. When these parameters are in their respective optimum ranges will the ratio d/h be maintained throughout the service life of the tread.